This research project is a continued, systematic investigation of primary hyperparathyroidism. Appreciative of classical clinical landmarks of the disease, the proposal combines an historical approach with modern analytical techniques of clinical medicine. Accurate decisions about the patient with primary hyperparathyroidism are uncertain because many outstanding questions about its natural history, pathophysiology and the extent to which an asymptomatic patient may show evidence for bone disease after more detailed evaluation are all unanswered. The research project is attempting to improve our understanding in these areas with a standardized, longitudinal approach to a sufficiently large number of patients with primary hyperparathyroidism. Recent advances in the measurement of circulating calcium regulating hormones and in the quantitative evaluation of static, dynamic and densitometric properties of the skeleton are responsible for the five goals: 1. greater knowledge of the natural history of primary hyperparathyroidism; 2. clearer understanding of the pathophysiologic mechanisms of the disease and their prognostic value; 3. comparison of complete evaluation with routine clinical assessment; 4. reversibility of the hyperparathyroid state; 5. predictive value of certain features regarding the development or prevention of complications. The ultimate goal of the research plan is to make the clinical approach to the patient afflicted with this disorder more secure as a result of greater overall understanding.